The phorbol diesters, e.g. phorbol 12-myristate 13-acetate (PMA), are extremely potent tumor promoters in mouse skin. In chick embryo fibroblasts (CEFs) in vitro, PMA at nanomolar concentrations induces a number of the same changes as those brought about by infection with Rous sarcoma virus (RSV). These changes include elevated production of plasminogen activator, stimulation of growth and deoxyglucose uptake, and decreased levels of the transformation-sensitive surface protein LETS. We propose to examine in detail the mode of action of PMA in CEFs. The studies have two overall objectives: to shed light on the phenomenon of tumor promotion and to clarify the control mechanisms of transformation-sensitive cellular properties. The structure-activity relationships of phorbol derivatives in CEFs will be compared with their activities as inflammatory and tumor promoting agents in mouse skin. The stability of the derivatives under tissue culture conditions will be examined to determine the role of degradation in modifying these relations. A major effort will be made to identify the putative receptor for the phorbol esters. The binding studies will use high specific activity radioactive derivatives other than PMA. If a receptor can be identified, its purification will be undertaken. Moreover, mutants insensitive to the phorbol esters will be isolated to provide genetic support for the putative receptor. To better determine the degree to which the phenotypes induced by PMA and RSV resemble each other, comparison of the biochemical and cellular changes induced by these agents will be continued. Particular attention will be directed at the surface changes induced by PMA.